Aerodynamic sail construction



1967 w. c. ROSS 3,356,059

AERODYNAMIC SAIL CONSTRUCTION Filed NOV. 18, 1966 INVENTOR BY fmzwwn wad ATTORNEYS United States Patent 3,356,059 AERODYNAMIC SAIL CONSTRUCTION Wallace C. Ross, 13 Academy Lane, Bellport, N.Y. 11713 Filed Nov. 18, 1966, Ser. No. 595,397 3 Claims. (Cl. 114-103) This invention relates to a new and improved spinnaker for sailing craft which will retain optimum aerodynamic shape when subjected to the stresses imposed by the wind.

A further object of this invention is to provide simultaneously an eflicient airfoil, yielding minimum friction with the wind, for sailing on a reach.

Another object of this invention is to maximize the drag of the spinnaker in the air flow on a dead run, thereby maximizing the forward drive on the sailing craft.

Another object of this invention is to provide a spinnaker which may be easily recut or altered by a sailmaker to meet particular requirements.

Spinnakers constructed according to traditional methods are subject to distortion of shape under the load of sailing. Such deformation has been a troublesome problem for sailors.

The effectiveness of a spinnaker depends not only on the drag it develops in the wind on a dead run but also on the airfoil effect it develops on a reach. Because of this airfoil effect, a low pressure area develops on the forward surface of the spinnaker, increasing the forward drive. It is therefore essential that the sail maintain an efficient aerodynamic shape.

Spinnaker sailcloth, such as nylon, may transmit load along either the warp of weft of its perpendicular thread lines without deformation of the fabric. However, stresses along the bias of the cloth, i.e. diagonally across the thread lines, will temporarily distort the cloth in the way an unbraced square may be distorted into a parallelogram. The consequent distortion of the sail shape results in separation of the air flow on the leeward surface of the sail, reducing its efficiency as an airfoil. In addition, the distortion may produce deep pockets of dead air on the windward surface of the spinnaker, reducing the effective drag obtainable on a dead run. The traditional methods of spinnakercon'struction from horizontal panels either with or without mitres or center. seams suffer from this distortion because of bias stresses at either the leech and luff or the center fold.

In carrying out my present invention, I provide a set of shaped panels so arranged in vertical and horizontal juxtaposition as to transmit the load on the sail along the thread lines 'of the sail material, thereby minimizing bias stresses and distortion of aerodynamic shape. In addition, the arrangement yields an eflicient airfoil and minimizes friction.

In earlier research, I discovered that the wind in a spinnaker travels in horizontal planes from luff to leech on a reach and from the center fold to both leeches on a dead run. In other Words, the primary air flow is horizontal rather than vertical as often propounded. I have also found that the greatest bias stresses in former spinnaker construction occur in the region of compound curvature, that is, the upper portion of the sail in which there is substantial curvature in both the horizontal and vertical planes. I further found that a compound curvature of hemispherical shape provides maximum drag in the air flow on a dead run, and, therefore, maximum drive for the sailing craft. My invention contemplates the optimum use of vertical and horizontal panels of sailcloth based on these facts.

In the accompanying drawings.

FIG. 1 is a diagrammatic view of the front of a spinnaker showing one arrangement of vertical and horizontal panels embodying my invention;

3,356,959 Patented Dec. 5, 1967 FIG. 2 is a side perspective view of a spinnaker under sail embodying my invention;

FIG. 3 is a diagrammatic View of spherical type gores joined to a horizontal panel illustrating continuity of the thread line of the warp and weft of the sail fabric there between as contemplated in my invention.

The embodiment of my invention illustrated in the accompanying drawings consists of a spinnaker with head 2, foot 3, leech-luffs 6, 7 and clews 13. The spinnaker is comprised of two portions, an upper portion of substantantially compound curvature consisting of vertical gores 1, and a lower portion of substantially horizontal curvature consisting of horizontal panels 8.

As the accompanying drawings illustrate, the embodiment of my invention comprises vertical panels or gores 1 tapered from a minimum width at the head 2 to a maximum width towards the foot 3, in the region of compound curvature, in order to transmit the load from the head 2 along the thread lines of the warp and weft of the sail fabric 1a. The gores 1 may be shaped with arcuate, vertical edges 4 to produce in combination a desired compound curvature, preferably spherical. It is advantageous to restrict the areas made from vertical gores to the region formerly subject to greatest bias stresses. This region of substantially complex curvature may comprise up to approximately 50% of the sail area, depending upon whether the sail is designed for light or heavy air, large or small craft.

In a spinnaker constructed wholly from vertical panels from the head 2 to the foot 3, frictional forces develop between the seams 4 joining the vertical panels 1 and the horizontal air flow 5 from luff 6 to leech 7, producing unwanted drag on a reach. More important, the vertical panels in horizontal cross-section do not produce an optimum aerodynamic curve. Rather, they produce a horizontal sequence of straight sections which may have a tendency to scallop. The horizontal panels form, on the other hand, a continuous airfoil for the wind.

The optimum spinnaker must not only transmit stresses along the warp or weft of the sailcloth, but also provide the most efiicient aerodynamic curve for the horizontal flow of air.

Thus, in the lower portion of the sail where curvature is substantially in the horizontal plane only, and bias stresses at a minimum, the spinnaker is comprised of horizontal panels 8 joined either with or without one or more mitres. The horizontal panels may also be shaped to yield a desired surfac in combination. For instance, the horizontal panels 8 may be shaped to yield a slight compound curvature themselves as in FIG. 2. The major curvature in this portion will be in the horizontal plane only, however.

Thus, the illustrated embodiment of my invention provides vertical gores 1 in the region ofsubstantial compound curvature joined at the lower portion to a set of horizontal panels 8. The panels are joined at the seams 4, 9, 10 by stitching from nylon, dacron or similar material, adhesive bonding, electric welding, heat sealing, or any other method used by sailmakers. Dacron stitching is preferable. In addition, the outer perimeter of the spinnaker is reinforced by a seam 11, similarly formed, in accordance with standard practice. Grommets 12 at each clew 13 and the head 2 provide means for securing the halyard 14, and sheets 15 in accordance with standard practice.

Although the horizontal panels may be shaped, it is important that the upper edge 10 of the upper horizontal panel 8 join the vertical panels 1 in such a way as to provide continuity of the thread lines of the warp and Weft of the sailcloth 1a, 8a from the vertical to horizontal portions and eliminate bias stresses in the area where they meet. This can be accomplished most easily by forming a straight horizontal edge 10 on the upper horizontal panel 8 parallel to either the warp or the weft of the sail fabric 8a, and a straight horizontal edge at the base of vertical panels 1, similarly parallel to either the warp or weft of the vertical panel sail fabric 1a. The two portions may then be joined so that the vertical seams 4 are perpendicular to the seam 10 at the upper edge of the upper horizontal panel with resulting continuity of the thread lines between the two portions, as in FIG. 3.

By such a combination, load from the head is distributed with minimum distortion, yet the horizontal panels present an eflicient airfoil to the horizontal flow of air and minimize friction.

The adaptability of the sail to alterations such as canting is readily apparent from the availability of both horizontal and vertical seams.

The ratio of the area of substantially compound curvature to the area of substantially simple curvature depends upon the dimensions and specifications for each particular spinnaker, so that the ratio of vertical panel area to horizontal panel area will vary. In one-design class sailboats the spinnaker specifications and dimensions are promulgated in the class rules.

The two areas of different curvature in any particular sail overlap, so that there is room for some variation in the ratio of horizontal to vertical panel area in that sail.

Furthermore, the number of panels used in each area may vary. Increasing the number of panels decreases the bias stresses up to a certain point, but the number is limited by the diminishing returns from increased workmanship and friction.

Ihave found that where class rules provide for a maximum girth or horizontal width which is significantly greater than the maximum permitted length along the foot 3, optimum characteristics of the spinnaker are retained by imparting a slight compound curvature to the horizontal panel area also. This can be accomplished by slightly tapering the ends of the horizontal panels with arcuate edges. Where, however, class rules specify a maximum girth dimension which does not differ substantially from maximum length along the foot 3, the horizontal panel area is substantially columnar and no shaping is necessary.

In three one-design sailboat classes we have found the following area ratios and panel numbers to be preferred.

In the Lightning Class, the class rules set a maximum limit on foot length of 7 feet 3 inches and maximum limit on leech-luff length of ZO-feet 6 inches. In addition the chord connecting a point 5 feet from the head on the leech and 6 feet from the head on the centerfold may not exceed 5 feet 10 inches. The chord connecting a point 10 feet from the head on the leech and 11 feet from the head on the centerfold may not exceed 8 feet 5 inches. With these dimensions a sail was constructed of nylon sailcloth having five horizontal panels and twelve vertical panels, joined at the seams by stitching with number 33 weight nylon thread. The vertical panels extended half the length of the leech-luff and the horizontal panels the other half. The horizontal panels were shaped by slight arcuate tapering at the ends to produce a slight compound curvature in the lower portion of the sail. The horizontal panels were constructed with a. single center seam or mitre. In the upper portion spherical gores were used to provide spherical curvature. The two portions were joined so that continuity of the thread line of the warp and weft of the sailcloth were maintained, in this case by using the preferred construction of a straight seam on the upper horizontal panel and a straight seam along the lower edge of the joined vertical gores.

For the Columbia 50, Cruising Club of America Rules set a maximum leech-luff length of 55.1 feet and a maximum girth or width from leech to luff of 32.58 feet. A sail for this type of cruising yacht with a luff length of 55 feet and maximum girth or width of 32.16 feet, was constructed from ten horizontal panels and twenty vertical panels of nylon sailcloth joined at the seams by number 33 weight nylon thread stitching. A single mitre or center seam construction was used in joining the horizontal panels.

For the 5.5 Meter Class, the class rules set a maximum limit on luff length of 29.1 feet and foot length of 22 feet. The maximum girth or width may not exceed 25 feet. A reaching spinnaker was constructed according to these dimensions from six horizontal and sixteen vertical panels of nylon sailcloth joined at the seams by number 33 weight nylon thread stitching. Again, single mitre construction was used for the horizontal panels.

In both the Columbia and the 5.5 meter spinnakers, the vertical gores extended half way down the length of the leech-luffs and the horizontal panels the other half.

I claim:

1. An improved aerodynamically shaped spinnaker comprising:

an upper portion of substantially compound curvature consisting of a plurality of vertically disposed gores of sailcloth tapered with increasing width from a minimum at the head to a maximum toward the foot, said gores being shaped with a arcuate vertical edges to produce in combination substantially spherical compound curvature and connected along substantially vertical seams whereby stresses are transmitted along the warp or weft of the sailcloth; a lower portion of substantially simple horizontal curvature consisting of a plurality of horizontal panels of sailcloth connected along substantially horizontal seams; and a seam joining the two portions so that continuity of the thread line of the warp and weft of the sailcloth between the two portions is preserved.

2. An improved aerodynamically shaped spinnaker as set forth in claim 1 in which the horizontal panels are joined together by mitre construction.

3. An improved aerodynamically shaped spinnaker as set forth in claim 1 in Which the horizontal panels are shaped to produce a slight compound curvature.

References Cited UNITED STATES PATENTS 2,544,770 3/1951 Willis 114-103 2,909,142 10/1959 Hood 114-103 MILTON BUCHLER, Primary Examiner.

T. M. BLIX, Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PatentNo. 3,356,059 December 5, 1967 Wallace C. Ross It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below:

In the'heading' to the-printed specification, lines 3 and 4, "l3

Academy-Lane, Bellport, N.Y. 11713" should read Bellport, N.Y., Assignor to Hard Sails Incorporated, Islip, N.Y., a corporation of New York Signed and sealed this 3rd day of March 1970.

(SEAL) Attest:

WILLIAM E. SCHUYLER, JR.

Edward M. Fletcher, Jr.

Commissioner of Patents Attesting Officer 

1. AN IMPROVED AERODYNAMICALLY SHAPED SPINNAKER COMPRISING: AN UPPER PORTION OF SUBSTANTIALLY COMPOUND CURVATURE CONSISTING OF A PLURALITY OF VERTICALLY DISPOSED GORES OF SAILCLOTH TAPERED WITH INCREASING WIDTH FROM A MINIMUM AT THE HEAD TO A MAXIMUM TOWARD THE FOOT, SAID GORES BEING SHAPED WITH A ARCUATE VERTICAL EDGES TO PRODUCE IN COMBINATION SUBSTANTIALLY SPHERICAL COMPOUND CURVATURE AND CONNECTED ALONG SUBSTANTIALLY VERTICAL SEAMS WHEREBY STRESSES ARE TRANSMITTED ALONG THE WRAP OR WEFT OF THE SAILCLOTH; A LOWER PORTION OF SUBSTANTIALLY SIMPLE HORIZONTAL CURVATURE CONSISTING OF A PLURALITY OF HORIZONTAL PANELS OF SAILCLOTH CONNECTED ALONG SUBSTANTIALLY HORIZONTAL SEAMS; AND A SEAM JOINING THE TWO PORTIONS SO THAT CONTINUITY OF 